In a mass spectrometer using an atmospheric pressure ion source, such as an electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI), the atmosphere within the ionization chamber is maintained at substantially atmospheric pressure, whereas the atmosphere within the analysis chamber, which contains a mass separator (e.g. quadrupole mass filter) and an ion detector, needs to be maintained in a high vacuum state. Therefore, in such a mass spectrometer, the configuration of a multistage differential pumping system is normally adopted, in which one or more intermediate vacuum chambers are provided between the ionization chamber and the analysis chamber so as to increase the degree of vacuum in a stepwise manner. In such a mass spectrometer having the configuration of the multistage differential pumping system, an ion transport optical system, which may also be called the “ion lens” or “ion guide”, is arranged within each intermediate vacuum chamber. An ion transport optical system is a kind of device for transporting ions to the subsequent stage while focusing those ions (and accelerating or decelerating them in some cases) by the effect of a direct-current electric field, a radio-frequency electric field or both of these two electric fields.
In order to transport the ions while efficiently trapping them, ion transport optical systems with various structures and configurations have conventionally been used. In one popularly used form of the ion transport optical system, a number of electrodes are provided around or along the ion beam axis, in which two radio-frequency voltages whose voltages are inverted from each other by 180 degrees are applied to any two electrodes neighboring each other among those electrodes, and simultaneously, a different level of direct-current voltage determined for each electrode is superposed on those radio-frequency voltages, so as to trap and transport ions while repelling them from the electrodes. Representative examples of this form of ion transport optical system include: a multipole radio-frequency ion guide in which an even number of rod electrodes equal to or more than four are arranged around the ion beam axis; and a multipole radio-frequency ion guide in which virtual rod electrodes, each of which consists of a number of plate electrodes arranged in the direction of the ion beam axis, are used in place of the normal rod electrodes. Patent Literature 1 discloses an ion transport optical system having a structure called an “ion funnel”, in which a number of aperture electrodes each of which contains a circular aperture are arranged along the ion beam axis. Patent Literature 2 discloses still another type of ion transport optical system, called a “radio-frequency carpet”, in which a number of ring electrodes are formed in a substantially concentric pattern on a printed circuit.
In the various aforementioned kinds of ion transport optical systems, the radio-frequency electric field created by applying radio-frequency voltages to a number of electrodes produces the effect of repelling the ions from those electrodes. This effect can be explained by employing the concept of “pseudo-potential” created by the oscillating electric field. A pseudo-potential is a kind of potential which acts on the secular motion which is the average of the microscopic oscillations caused by the oscillating electric field. Macroscopically, the ions move as if they are subjected to a repulsive force from the electrodes proportional to the gradient of the pseudo-potential. Accordingly, in generally used ion transport optical systems using a radio-frequency electric field, the collision of the ions with the electrodes is prevented by this pseudo repulsive force while the ions are being focused and transported in the desired direction by the direct-current electric field superposed on the radio-frequency electric field.
The ion funnel and the radio-frequency carpet mentioned earlier as conventional examples are characterized in that the efficient trapping and transporting of the ions is realized by using a dense arrangement of micro-sized electrodes. To this end, a large number of micro electrodes need to be arranged with a high level of positional accuracy. Additionally, direct-current voltages having different voltage values need to be applied to the respective micro electrodes in addition to the radio-frequency voltages. Therefore, it is difficult to reduce the related cost, and the device cost also tends to be high. Additionally, in many cases, the electrodes need to be arranged so as to entirely surround the area where the ions pass through. This requirement poses various challenges in creating a small-sized apparatus or changing the structure of the apparatus. Thus, there has been strong demand for an ion transport optical system which requires a smaller number of electrodes than the conventional ion funnel or radio-frequency carpet and yet one which is capable of achieving the same levels of ion-trapping and ion-transporting efficiencies as those of the conventional apparatuses, as well as one which has such a simple structure that allows flexible changes in the structure of the apparatus.